The invention relates to a catalyst and, more particularly, to a catalyst including a spinel containing copper (Cu), zinc (Zn), and aluminum (Al). The invention further relates to a process for the production of the catalyst and, more particularly, to the process for producing a catalyst for gases, where for producing the catalyst there is selected a spinel containing the metals copper (Cu), zinc (Zn) and aluminum (Al).
German Patent document DE 43 01 470 A1, which is not a prior publication, discloses a CuAl.sub.2 O.sub.4 spinel which, with the additions of tin, lead, an element of the main or transition group II of the Periodic Table of the Elements as an oxide or salt or in elemental form and subsequent calcination, is combined to give a spinel. The previously known spinel of this type is used for the decomposition of N.sub.2 O. The use of this catalyst for catalytic oxidation of CO.sub.2 and NO.sub.x, in particular at temperatures of several 100.degree. C., is not known.
The need to purify gases, particularly engine exhaust gases, by removing nitrogen oxides therefrom, i.e., the degradation of NO.sub.x, for reasons of environmental protection is an urgent problem. Engine exhaust gases which must be purified, besides lowering the concentrations of CO, etc., are typically formed, for example, in internal combustion engines and here particularly in diesel or lean-mix engines.
EP 042 471 B1 discloses a catalyst for the catalytic oxidation of carbon monoxide (CO) to carbon dioxide (CO.sub.2). The catalyst contains the metals copper (Cu), zinc (Zn) and aluminum (Al) as metal oxides. At least 60% of the Cu present is bound to the aluminum oxide as a copper oxide-aluminum oxide spinel. In the free spaces of the porous spinel, zinc oxide is present in a proportion by weight of from 1% to 20%.
However, the catalyst previously known from EP 042 471 B1 is only known to have a purifying action with respect to CO which is based on a catalytic oxidation of the CO. Whether the catalyst is likewise suitable for purifying gases containing NO.sub.x and/or hydrocarbons, as are formed, in particular, in the above-mentioned internal combustion engines and, for example, also in thermal power stations and the like, is not known.
There is therefore needed to further develop the known catalyst so that it has a good purifying action, in particular in the case of NO.sub.x, together with very good stability to the constituents of waste gases. It is further needed to develop a process for producing such a catalyst.
In the case of a catalyst of this type, these needs are met according to the present invention by a catalyst including a spinel containing copper (Cu), zinc (Zn), and aluminum (Al). The catalyst used for the catalytic reduction of NO.sub.x and/or the oxidation of hydrocarbons in waste gases contains a copper oxide-zinc oxide-aluminum oxide spinel of the chemical formula Cu.sub.A Zn.sub.C Al.sub.D O.sub.4, where: A+C+D=3 and A&gt;0, C&gt;0 and D&gt;0.2. These needs are also achieved by the process for producing a catalyst for gases, where for producing the catalyst there is selected a spinel containing the metals copper (Cu), zinc (Zn) and aluminum (Al). For producing the catalyst of copper oxide and cobalt oxide and zinc oxide and aluminum oxide a spinel is prepared containing all four metals. The use of a Cu.sub.A Zn.sub.c Al.sub.D O.sub.4 spinel as a catalyst makes the catalyst largely stable to the constituents of the waste gases such as, for example, H.sub.2 O, NO.sub.x, CO.sub.2 and/or SO.sub.2. The spinel has an oxidizing effect on hydrocarbons and a catalytically reducing effect on the NO.sub.x. The reduction of NO.sub.x takes place in oxygen-containing gas and in the presence of reducing agents such as, for example, the hydrocarbons mentioned above. Advantageously, hydrocarbons are present in a sufficient concentration in the exhaust gases of internal combustion engines. In favorable cases, an amount of reduction above 60% can be achieved at temperatures above 300.degree. C.
Furthermore, the present invention will be described by means of an example, the measurements of which are presented graphically in the accompanying Figures.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.